Square optical pulse generator

ABSTRACT

An all-optical fibre Sagnac antiresonant interferometer (2) is formed from an optical fibre having a non-linear refractive index. The coupler (8) is a dichroic coupler coupling equal portions of a cw optical signal at 1.53 μm from laser (16) to ports (14 and 16) and all of a pulsed optical signal from laser (20 ) to port (14). The intensity of the pulsed optical signal is sufficient to provide a relative phase shift in the counter propagating 1.53 μm signals. The loop (6) is longer than the inverse of the absolute difference in group delays of the cw and pulsed optical so causing a square optical pulse at 1.53 μm to be switched to port (12) of the coupler.

This invention relates to square optical pulse generators.

Considerable effort has been employed in generating square opticalpulses for all-optical switching purposes using spatial transformtechniques as discussed for example in an article by A. M. Weiner et alentitled "Femtosecond Pulse Tailoring" published in Optics Letters 13,300 (1988).

This method requires suitable mask to be made which is an involvedprocedure and the transformation must be carried out in bulk opticswhich is inconvenient when the square pulses produced are to be used inoptical fibre devices or networks.

According to the present invention an optical pulse generator comprisesa first optical coupler having a first and a second pair of opticalcommunication ports in which substantially equal first signal portionsof an optical signal at a first wavelength received at a port of onepair are coupled to the two ports of the other pair of ports;

an optical waveguide coupling together the second pair of ports havingan interaction section which includes a material having a non-linearrefractive index;

a cw optical source for providing a cw optical signal at the firstwavelength optically coupled to a first port of the first pair of ports;

a pulsed optical source for providing a pulsed optical signal at asecond wavelength optically coupled to the interaction section so thepulsed optical signal propagates along it in substantially onedirection;

the intensity of the pulsed optical signal being sufficient to provide arelative phase shift in the first signal portions as they propagateround the optical waveguide and the interaction section being longerthan the inverse of the absolute difference in group delays of the cwand pulsed optical signals.

In this specification by "non-linear" we means that the refractive indexof a material varies with the intensity of the transmitted signal.Typically the refractive index n is given by the formula n=n_(o) +n₂/E/² where n_(o) is the linear refractive index, n₂ is the Kerrcoefficient and /E/² the intensity of the transmitted signal.

In the absence of the pulsed optical signal the first optical couplerand the optical waveguide, which form a Sagnac antiresonantinterferometer, act as a loop mirror to the cw optical signal in thatthe signal entering the coupler at the first port will be reflected i.e.it will exit form the same port. This is because the twocounter-propagating portions maintain the same relative phase. When thepulsed optical signal propagates along the interaction section of thewaveguide-so inducing a phase shift in the first portion whichco-propagates with it in the same direction-the condition for reflectionis broken and some of the cw optical signal will exit the second port.The present invention relies on the realisation that the group delays ofthe cw and pulsed signals are different so that for a sufficiently longinteraction section the pulse can effect switching to the other portpulse of cw optical signal wider than the width of the pulse from thepulsed source. Thus a short pulse at the second wavelength can be usedto provide a square optical pulse at the first wavelength which willhave a rise and fall time of the order of the short pulse and a widthdependant on the length of the interaction section.

Preferably, the first optical coupler is a dichroic optical fibrecoupler substantially all of the pulsed optical signal received at oneport of one pair to one port of the other pair and the optical waveguideis an optical fibre. The pulsed optical source being optically coupledto the first port as this provides in a simple manner both the twocounter propagating cw portions at the first wavelength and propagationof the pulsed signal in a single direction around the optical fibre.

The pulsed optical source can be coupled to the interaction portion byother arrangements, for example by means of dichroic couplers in theoptical fibre loop.

Other waveguides providing the necessary non-linearity may also beemployed within the scope of the present invention.

Embodiments of the present invention and its principle of operation willnow be described in more detail with reference to the accompanyingdrawings in which -

FIG. 1 is a schematic diagram of an experimental square optical pulsegenerator according to the present invention;

FIG. 2 is a graph of the calculated non-linear phases imposed on the cwoptical signal by an optical pulse for different lengths of interactionsection; and

FIGS. 3a-d are photographs of square pulses generated by the apparatusof FIG. 2 for different powers of pulsed optical signal.

Referring to FIG. 1 a Sagnac antiresonant interferometer 2 is defined bya single silica optical fibre 4 formed into an optical fibre loop 6 withpotions of the optical fibre being formed into a fused optical fibrecoupler 8 having a first pair of ports 10, 12 and a second pair of ports14, 16. In this embodiment the loop 6 provides an interaction section byproviding an optical fibre exhibiting a non-linear refractive index. Thefibre loop 6 was 500 m long and polarisation maintaining.

A modelocked Nd:YAG laser 20 provides a pulsed optical signal of the 130ps pulse width at 1.3 μm which is coupled into the first port 10 bymeans of a dichroic coupler 18.

A continuous wave (cw) F-centre laser 16 provides a cw optical signal at1.53 μm which is also coupled to the port 10 of the coupler 8 by meansof the optical couplers 18 and 22.

The coupler 8 is manufactured in well known manner so as to couple equalportions of the cw optical signal coupled to port 10 to the ports 14 and16 to produce two counterpropagating, equal intensity cw portions in theloop 6 and to couple substantially all of the pulsed optical signal intoport 14 (an extinction ratio of 37 dB at 1.3 μm) so the pulsed signalpropagates in only one direction round the loop 6.

The coupler 22 has a 50:50 splitting ratio at 1.53 μm and is included inthis experimental arrangement to provide a monitoring point for the backreflected signal. The coupler 18 is a dichroic coupler in which both the1.53 μm and 1.3 μm optical signals are combined.

The performance of the device was monitored at coupler 22 with aphotodiode 24 which had a pulse response of 70 ps FWHM and the outputdisplayed on a sampling oscilloscope (not shown). The signal at thismonitor diode is the 1.53 μm signal alone because none of the 1.3 μmpulsed optical signal returns to this port. The average power of the 1.3μm signal in the loop is measured at the free output port 12 of coupler8.

Consider now when a cw optical signal at 1.53 μm and a pulsed opticalsignal at 1.3 μm are propagating round the loop 6. Under theseconditions the portion of the cw signal co-propagating in the samedirection and with the pulsed optical signal can be described by thefollowing pair of coupled equations in normalised units. ##EQU1## Inequations 1, A is the pulse signal (high power) and B is the cw signal(low power) which is propagating in the same direction as A. The groupdelay is given by β'_(A) and β'_(B) for the appropriate waves. Since Bis small we can neglect terms of order B². In addition, if we transforminto the frame moving with the group velocity of the B wave thenequation (1) becomes ##EQU2## is the difference in group delays of thetwo waves. Note, we have also neglected the SPM of the pulse signalsince the nonlinear response is unaffected by this term.

The solution for A is simply a traveling wave given by ##EQU3##

The equation for B can now be integrate to give ##EQU4## where L is thelength of the loop 6. Equation (5) is exact even when SPM is included inthe pump. The expression in brackets in equation (5) represents thephase change φ of the CW B signal caused by the pulse signal P_(a) (t).The reflected B signal from the loop mirror can be simply expressed interms of this phase as

    B.sub.ref =B(1+cos (φ))B.sub.in /2                     (6)

(see N. J. Doran and D. Wood, "Non-linear Optical Loop Mirror" OpticsLett 13 56-58 (1988).). This expression shows that the low power Bsignal is modulated by the high power A signal pulse. In addition, thedifference in group delay between the two signals leads to a broadeningof the reflected pulse because the phase φ depends upon the integral ofthe pump pulse P_(A) (t). As an example, if the A signal is given by

    A(t)=U sech(t)                                             (7)

then the nonlinear phase change is given by

    φ(t)=U.sup.2 (tanh(t)-tanh(t)-Δβ'L)/Δβ'(8)

Referring to FIG. 2 there is shown the nonlinear phase for two looplengths. FIG. 2a is the result for a loop length which is small comparedto the pulse walk off length 1/(Δβ'). The nonlinear phase is similar tothe SPM result in which the phase is proportional to the pump pulseshape. In FIG. 2b the loop length is large compared to the walk offlength. The peak phase change is larger, since we are now utilising allthe available interaction length, and is constant between the leadingand trailing edge so forming the required square optical pulse.

The results for a number of different pump powers are shown in FIG. 3.At an average (peak) power of 20 mW (2W) at 1.3 μm (FIG. 3a) we observecomplete switching of the 1.53 μm probe signal. However, inspection ofFIG. 3a shows the 1.5 μm pulse to be flat topped with a width of 300 ps.This is due to the difference in group delay between the two signals asexplained above. As the pump power is further increased (FIGS. 3b,c) westart to observe the periodic intensity response of the nonlinear loopmirror. In FIG. (3b) the central part of the pulse is now fullyreflected. However, the leading and trailing edges which should gothrough a stage of zero reflection appear not to. This is merely due tothe limited response of the photodiode. The photodiode response time isnot a problem in the central part of the pulse since the flat region ofphase (see FIG. 1b) is longer than this response time. Similarinstrument limited features can also be seen in FIG. (3c). The number ofcycles we can observe is limited by the onset of stimulated Ramanscattering which occurs here at an average power of 110 Mw. Any furtherincrease in power above this value does not produce any increase in peakpower at 1.3 μm but simply transfers power to the longer wavelengths.

We claim:
 1. A square optical pulse generator comprising:a first opticalcoupler (8) having a first and a second pair of optical communicationports (10, 12 and 14, 16) in which substantially equal first signalportions of an optical signal at a first wavelength received at a port(10) of one pair are coupled to the two ports (14,16) of the other pairof ports; an optical waveguide (6) coupling together the second pair ofports having an interaction section which includes a material having anon-linear refractive index; a cw optical source (16) for providing a cwoptical signal at the first wavelength optically coupled to a first port(10) of the first pair of ports (10, 12); a pulsed optical source (20)for providing a pulsed optical signal at a second wavelength opticallycoupled to the interaction section (6) so the pulsed optical signalpropagates along it in substantially one direction; the intensity of thepulsed optical signal being sufficient to provide a relative phase shiftin the first signal portions as they propagate round the opticalwaveguide (6) and the interaction section being longer than the inverseof the absolute difference in group delays of the cw and pulsed opticalsignals.
 2. A pulse generator as claimed in claim 1 in which the firstoptical fibre coupler is a dichroic optical coupler (8) couplingsubstantially all of the pulsed optical signal received at one port ofone pair (10, 12) to one port of the other pair (14, 16), the pulsedoptical source (20) being optically coupled to the first port (10).
 3. Apulse generator as claimed in claim 2 in which the pulsed optical signaland cw optical signal are combined by means of a second dichroic opticalcoupler (18) having an output port coupled to the first port (10) of thefirst optical coupler (8).
 4. A pulse generator as claimed in claim 2 inwhich the pulsed optical source (20) is coupled into the opticalwaveguide (6) by means of a dichroic coupler (8).
 5. A pulse generatoras claimed in claim 1 in which the optical waveguide (6) comprises anoptical fibre.
 6. A pulse generator as claimed in claim 1 in which eachoptical coupler (8) comprises an optical fibre coupler.